1. Field of the Invention
The present invention relates to osteosynthesis systems for the spinal column.
2. Description of Related Art
Document FR-2 659 546 discloses an osteosynthesis system for the spinal column or backbone which has anchor members for fixing to a series of vertebrae of the spine, and at least one rigid link element designed to interconnect the anchor members. Once the system has been fixed to a series of vertebrae and once it has been assembled, it constitutes a rigid device preventing the vertebrae from moving relative to one another. This rigidity enables the device to support all or part of the stresses instead of the series of vertebrae concerned. Nevertheless, the first vertebra adjacent to the series is quite free to move relative to the series of vertebrae made rigid. A sharp discontinuity in the distribution of stresses along the spine can then arise between the last vertebra of the series and the first free vertebra. Consequently, the disk between these two vertebrae is overtaxed, and indeed an abnormal increase in stresses is observed at the disk. Very often this accelerates degeneration of the disk. This can be referred to as the "new-hinge" syndrome.
Document WO 94/21185 presents an osteosynthesis device for the spinal column in which two pedicular screws are interconnected by a link member having a flexible central portion that is U-shaped and offset laterally. Thus, when the two vertebrae are subjected to stresses tending to displace the two anchor members in relative rotation about an deformation axis, the link member is itself subjected to a bending moment and it bends elastically about the deformation axis. It therefore supports these stresses in part. The system recreates in part the biomechanical organization of the functional unit made up of the two fixed vertebrae together with the intervertebral disk. Nevertheless, that device has the drawback that the possibility of relative rotation between the two pedicular screws is essentially a function of the elasticity of the intermediate portion. In other words, the suitability of the intermediate portion for deforming relies on said elasticity. Unfortunately, when said elasticity is high so as to restrict relative displacement, then the vertebrae can be subjected to high levels of stress which can be harmful to them. Conversely, when the elasticity is low, support provided to the vertebrae is mediocre.